Sip Uri
Yes, your sip URI will be
123456@us.voxalot.com.
But as you can see I use the us proxy, why? Simple my Voxalot number is registred at a softphone with the us Proxy.
So I think (I´m not shure) that your Sip URI will depend on where are your Voxalot number registred, unless they have theyer Network ( for users, of course) registred in another class the the 255.255.255.0. If they have it in a Class that permit differents Network see theyerselfs in this case does not matter what you will use as a proxy (Voxalot proxy, of course) you can use us, eu or proxy1, proxy2, because all them will work.
I hope this explanantion can help you as my english is not so good.